What nosing projection is and why code caps it
Nosing projection is the distance the front edge of a tread sticks out past the face of the riser below it, and it exists to give a little extra effective tread depth for the ball of your foot without needing a deeper, more expensive stair overall. The trouble is, too much projection creates a ledge that a descending foot can catch its toe under, which is exactly the trip mechanism codes are trying to prevent, so both US and UK regulations cap how far a nosing is allowed to overhang.
US IRC R311.7.5.3 caps nosing projection at 1.25 in (32 mm) with a minimum of 0.75 in (19 mm) required on stairs with solid risers where the tread depth is less than 11 in, though a nosing isn't required at all where the tread depth is already 11 in or more. UK Approved Document K takes a broadly similar approach, generally expecting a nosing of around 16-25 mm on stairs where the going is on the tighter side, without a hard universal minimum in the way the US code sets one, but with the same underlying trip-hazard logic driving the cap.
How nosing interacts with your tread going
A nosing calculator becomes genuinely useful once you realise that projection and true tread going trade off against each other in the code's minimum going calculation. If your code requires a 10 in (25 cm) minimum going and your actual horizontal tread board is only 9.25 in (23.5 cm) deep, a 0.75 in (19 mm) nosing that overhangs the riser below brings your effective going up to the required 10 in, which is precisely why nosing exists as a design tool rather than just a decorative detail. Push the nosing further, say to the maximum 1.25 in (32 mm), and you could in theory use an even shallower physical tread board while still meeting the effective going minimum, though most designers stop well short of the maximum for comfort and to avoid the toe-catch risk that comes with an aggressive overhang.
Open riser stairs use a different rule entirely
On open riser stairs, US code instead requires that a 4 in (10 cm) sphere cannot pass through the gap between treads, which effectively limits how far back a tread's leading edge can sit rather than regulating overhang projection in the same way as closed-riser stairs.
How much runner do I need?
Nosing projection is exactly the figure a stair runner needs to wrap around at the front of every tread, so getting it right changes your total length noticeably across a full flight. Take a US example with a deliberately generous 1 in (25 mm) nosing rather than the more typical 0.75 in: a 13-step flight with 10 in tread run and 7.5 in riser height now needs 10 + 7.5 + 1 = 18.5 in per step instead of 18.25 in, giving 240.5 in across 13 steps, or 20.04 ft. Add the usual 24 in finishing allowance for 22.04 ft, then a 7% waste margin brings the total to roughly 23.6 ft, about 3-4 in more runner than the 0.75 in nosing version across the whole flight.
In UK metric terms, the same flight with a 25 mm (2.5 cm) nosing instead of 19 mm gives 25 + 19 + 2.5 = 46.5 cm per step, or 604.5 cm across 13 steps. Add 60 cm finishing allowance for 664.5 cm, then a 7% waste margin brings the total to roughly 7.11 m, again a small but real difference over the 19 mm nosing figure. On a longer flight, say 20 steps, that same nosing difference compounds to closer to 5-6 in (13-15 cm) of extra runner needed, which matters if you're ordering close to the exact metre.
| Nosing depth | Per-step total (US) | 13-step subtotal (US) | 13-step subtotal (UK) |
|---|---|---|---|
| 0.75 in / 19 mm | 18.25 in | 19.77 ft | 5.97 m |
| 1.0 in / 25 mm | 18.5 in | 20.04 ft | 6.05 m |
| 1.25 in / 32 mm (max) | 18.75 in | 20.31 ft | 6.13 m |
Know your numbers in 30 seconds
Enter your step count, tread depth and riser height — the stair runner calculator gives you the exact length to order, in metres and feet, with waste included.
Open the stair runner calculatorMeasuring nosing on an existing staircase
If you're measuring an existing flight rather than designing a new one, hold a tape or set square against the vertical face of the riser and measure horizontally out to the front edge of the tread above it; that gap is your nosing projection. It's worth measuring two or three steps rather than just one, since older staircases, particularly hand-built Victorian and Edwardian flights, often have small inconsistencies of a few millimetres between steps that a factory-built modern stair wouldn't have. Take the largest nosing figure you find on the flight for your runner calculation, since under-allowing for wrap on even one step will leave that step's runner sitting slightly proud or loose at the front edge.
- Measure nosing projection on at least three steps, not just one, on older or hand-built stairs.
- Use the largest nosing figure found on the flight for your runner wrap allowance.
- Check whether any steps have a rounded bullnose profile, which needs slightly more wrap than a square nosing.
- Confirm your local code's nosing cap before finalising a new-build stair design.
- Remember open riser stairs use a sphere-gap rule instead of a nosing projection limit.
Nosing profile and how it affects runner fit
A square-edged nosing needs the least extra runner to wrap cleanly, while a rounded bullnose nosing, common on older and higher-end staircases, needs a touch more fabric to follow the curve without pulling tight or leaving a visible gap underneath. A pencil-round or chamfered nosing sits between the two. If your stairs have a pronounced bullnose, add an extra 0.25-0.5 in (6-13 mm) per step to your wrap allowance beyond the flat projection measurement, since the curved surface area is genuinely greater than the straight-line projection distance alone would suggest.
Whatever the nosing profile, run your final measurements through a stair runner calculator rather than eyeballing an average across the flight, because nosing differences that look trivial on a single step add up to real extra length once multiplied across 13, 16 or 20 steps, and that's the single most common reason a runner order comes back a few inches short at delivery.
Whatever you decide, get the numbers first: the stair runner calculator doubles as a stair runner length calculator, adding nosing wrap, landings, winders, pattern repeat and waste so you order the right length the first time.
Still deciding on material, width or pattern? Our hub on stair runners compares weaves, widths, backings and fitting styles side by side, then hands the numbers straight to the calculator.
Frequently asked questions
What is the maximum stair nosing projection allowed?
US IRC code caps nosing projection at 1.25 in (32 mm), with most residential stairs using between 0.75 in and 1 in (19-25 mm) in practice. UK guidance sits in a similar range, generally 16-25 mm, without one single fixed universal maximum.
Is a nosing required on every staircase?
No. US code doesn't require a nosing where the actual tread going is already 11 in (28 cm) or more, since the nosing exists specifically to boost effective going on shallower treads, not as a mandatory decorative feature.
How do I measure nosing projection on my own stairs?
Hold a tape measure horizontally from the vertical face of the riser out to the front edge of the tread above it. Measure a few steps rather than just one, since older hand-built stairs often vary slightly step to step.
Does nosing depth affect how much stair runner I need?
Yes. Nosing is part of the per-step wrap allowance in a runner length calculation, so a deeper nosing genuinely adds a small but real amount of extra length across a full flight, especially noticeable on longer runs of 15-20 steps.
What is the rule for nosing on open riser stairs?
Open riser stairs are checked differently, using a rule that a 4 in (10 cm) sphere must not be able to pass through the gap between treads, rather than a nosing projection limit used on closed-riser stairs.
Does a rounded bullnose nosing need more runner than a square nosing?
Yes, a rounded bullnose needs slightly more fabric to wrap the curved surface cleanly than a flat, square-edged nosing of the same projection depth. Add roughly 0.25-0.5 in (6-13 mm) extra per step for a pronounced bullnose.
Can too much nosing projection be a trip hazard?
Yes, an overly deep nosing creates a ledge that a descending foot can catch under, which is exactly why codes cap the maximum allowed projection rather than leaving it unlimited.
Why do old staircases sometimes have inconsistent nosing depths?
Hand-built older staircases, particularly Victorian and Edwardian flights, were often cut to eye by a joiner rather than to a precise jig, so small variations of a few millimetres between steps are common and worth measuring individually before ordering a runner.
Know your numbers in 30 seconds
Enter your step count, tread depth and riser height — the stair runner calculator gives you the exact length to order, in metres and feet, with waste included.
Add your nosing wrap into the stair runner calculatorEvery guide is written from hands-on measuring and fitting experience, checked against the calculator’s own maths, and re-reviewed whenever prices or methods change. Read our editorial standards.